Recently, personal computers ("PCs") used a variety of techniques for providing input and output. These included a serial port, usually an RS232, a parallel port and several ISA expansion slots included on the mother board. Connecting the PCs to anything more complicated than a mouse, modem or printer, required the lid to be removed and dip switches to be set and software configured. SCSI (small computer systems interface) permitted access to external storage devices, but required a large cable connector with the need to manually set ID numbers and have a terminator.
Referring to FIG. 1, Universal Serial Bus ("USB") is specified to be an industry standard extension to the PC architecture with a focus on computer telephony interface, consumer and productivity applications. The USB architecture provides for ease of use of peripheral expansion, transfer rates up to 12 megabits per second, protocol flexibility for mixed modes, isochronous data transfers and asynchronous messaging. USB is a cable bus supporting data transfer between the host PC and a range of simultaneously testable peripherals. One host controller can support up to 127 physical node devices using a tiered topology. A hub is at the center of each star with each wire segment creating a point-to-point connection of up to 5 meters. The 5 meter limitation may be between a host and a hub or a hub function or a hub connected to another hub or function.
Alternatively, a number of peripherals can be daisy-chained together via the 4-wire USB cable. One of the 4 wires referred to as VBUS provides a DC voltage of +5 volts and another wire provides a ground signal. The USB cable provides power to the devices along the chain. Signaling takes place over two wires between two end points. The signals of each end point are driven differentially over a 90 ohm impedance with each receiver featuring input sensitivity of at least 200 millivolts. A non-return to zero invert (NRZI) with bit stuffing to insure adequate transitions is used to carry the bus clock down the chain. A sync field precedes each data package to allow the receivers to synchronize their bit recovery clocks. The serial interface provides a maximum bandwidth of 12 megabits per second and can connect as many as 127 node devices to a host system.
Physically, USB devices ranging from a mouse, joystick to telephones connect to a host via layers of multiport hubs. The requisite hub called "the root hub" is located in the host and can include multiple ports. Hubs linked to USB devices via point-to-point connections. The host views all USB devices as if they connect in a true star arrangement. USB supports both the standard devices that operate a full 12 megabit rate and low end devices that use only a 1.5 megabit sub channel. Hubs must always support 12 megabit operations and must insure that 12 megabit transmissions do not go to 1.5 megabit devices.
USB cables that carry 12 megabit traffic require a shielded twisted pair construction on the signal pair and can be no longer than 5 meters. Low speed cables can be no longer than 3 meters and require no shielding or twisting of the signaling pair. The host uses a master slave protocol to control the bidirectional communications with USB devices. The interface employs a 1 KHZ bus clock that instigates bussing a new frame every 1 millisecond. The interface handles multiple transactions including time critical isochronous transactions within each frame in 1 millisecond period and 12 megabit per second bandwidth limit type of isochronous data streams that the interface can successfully carry.
One problem with the universal serial bus is that it provides only one voltage. Devices that operate at different voltages or have high power requirements are required to supply their own voltage sources and power sources. In some environments, for instance, the retail point-of-sale environment, this additional cabling for power creates a non-aesthetic appearance at the store front.
A problem with providing an enhanced USB signalling is that an end-user may attempt to couple a standard USB plug into the enhanced USB receptacle.